Disposable pressware containers, such as plates, trays, bowls, and the like made from paperboard and other suitable materials are sometimes manufactured on an inclined die set. In a typical forming operation, a web of stock is fed continuously from a roll through a cutting die to form circular blanks. The cut blanks are then fed, typically by gravity, into position between upper and lower die halves of a die set, which is sometimes referred to herein as the forming apparatus.
As illustrated in FIG. 1, a typical die set 40 includes an upper die half 20 that opposes a lower die half 22 in a facing relationship. FIG. 1 is a simplified depiction of die set 40 for illustrative purposes. The upper die half is mounted for reciprocating movement in a direction that is typically inclined with respect to the vertical plane. A typical die set for forming pressware containers includes a male or punch die half, such as upper die half 20, and a female die half, such as lower die half 22. One or more portions of the die halves may be spring-biased such as by use of springs 25. The upper die half and the lower die half press the blank, which can be a circular paperboard blank, into the shape of a plate or other desired pressware.
In some instances, the lower die 22 has a base portion (such as base portion 30 shown in FIG. 1), a knock out (such as knock out 27 shown in FIG. 1) and a removable contour portion (such as contour portion 28 shown in FIG. 1). The upper die half 20 can similarly include a punch portion (such as punch portion 34 shown in FIG. 1) and a knock out (such as knock out 36 shown in FIG. 1). In some embodiments, the die set includes a pressure ring 24 and a draw ring 26. As discussed below, the draw ring helps control pleating during plate formation.
Paperboard plate stock is conventionally from about 0.01 to about 0.025 inches in thickness. With plate stock of such thickness, it is sometimes desirable to maintain spacing of less than 0.01 and 0.025 inches (slightly less than the thickness of the plate stock) between the pressure ring and the draw ring to create resistance that helps control the folds (pleating) evenly around the circumference of the plate. Specifically, the draw ring provides pressure on the outer circumferential area of the blank so that the blank is pulled onto the forming surface of the lower die half with some resistance.
To produce quality product at the desired rate, it is important to have consistent forming operations. In particular, it is important that the circular blank be properly positioned within the lower die half. A typical forming apparatus includes blank stops that are configured to stop a blank as it is fed into position between the upper and lower die halves. It is also desirable that the blank stops be configured to allow a container formed from such a blank to slide through and exit the die forming apparatus without obstruction.
As shown in FIGS. 2-4, a draw ring 26 includes at least one conventional blank stop 10 having a vertical member 12 that extends in a generally vertical orientation to stop the blank as it is fed into the forming apparatus. Such conventional blank stops are positioned with respect to the draw ring 26 using one or more spacers or shims 14, which are located between the blank stop 10 and the draw ring 26 and allow for the adjustment of the blank stops at discrete intervals. In this way, the blank stops can be positioned with respect to the forming apparatus to account for blank diameter variations, bounce, angled blank transfer chute delivery, blank curl, or other factors. However, adjustment of the blank stops is difficult because a bolt, such as bolt 15, has to be loosened and the appropriate shims inserted, which makes adjustment cumbersome and limited to the available shims. Moreover, the process can be dangerous because the operator inserting the shim is subjected to a burn risk from the forming apparatus, which is typically hot.
Blanks that are gravity fed to the die forming apparatus contact the fixed angular blank stops with significant speed and energy. As a result, blanks will often “bounce back” at least once before settling in between the two dies. If the blank is not centered between the two die halves, the product is formed off center, which results in a longer downturned edge on one side of the product when compared with the other side of the product. This differing downturned edge will often compromise product strength and impact visual stack aesthetics so the stacks do not look uniform. Thus, such off center formed products are rejected from the die set and discarded as waste.
Another alternative for catching the gravity fed blank is the use of rotating pin stops, such as pin stops 16 shown in FIGS. 5-8. These pin stops are attached to the draw ring die half (or elsewhere on the forming apparatus if a draw ring is not used) and the diameters of the pins are chosen to help center the blank in the die set. These pin stops may include adjustment screws 18 that allow the pin stops to be adjusted with respect to the forming dies. Such adjustment does not require the use of shims and allows for infinite adjustability in an inward/outward direction with respect to the die. In some configurations, as shown in FIG. 7, a set of two rotating pin stops per side (four pins total) may be used toward the front of the draw ring to stop the incoming blank. When the pin stops 16 are capable of spinning/rotating as shown in FIGS. 7-8, the rotation absorbs some of the kinetic energy of the gravity fed blanks and thus helps reduce blank bounce and off center forming of a non-uniform plate. The rotating pin stops also allow the blank to roll or move to an on center position more quickly, thus allowing consistent product formation at higher speeds.
In some embodiments, such as the one of FIG. 7, the back two pin stops are taller than the front two pin stops, so that the back two pin stops guide the blank as it is gravity fed into the die forming apparatus, the front two pins position the blank, and the formed plate can slide over the lower front two pin stops.
As described in U.S. Patent Publication No. 2007/0042072, which was filed on Aug. 18, 2006 and is incorporated herein by this reference, improvements have been made to conventional pressware forming machines to allow for at least one additional die pair in the pressware forming machine. This is achieved by narrowing the die pairs used in the machine so that an additional die pair fits within the machine. Because of the narrowing of the die pairs, only two rotating pin stops 16 (as opposed to four) are capable of fitting with each narrowed die pair, as illustrated in FIG. 8. Thus, the draw ring includes two rotating pin stops 16 and two non-rotating guide pins 17. Using two rotating pin stops instead of four rotating pin stops is not as effective at absorbing the kinetic energy associated with the gravity fed blank as it hits the rotating pin stops. Specifically, the inventors have found that there is undesired “bounce back” associated with the use of only two rotating pin stops, which can result in off center forming of plates, as described above. In addition, the pin stops are not as effective at stopping the gravity fed blank as the blank stop 10 shown in FIGS. 2-4. Specifically, when using the pin stops 16, the blank is capable of sliding across the top of one or both of the rotating pin stops as it is gravity fed into the die set.